1. Preheat oven to 425°F.
2. Whisk 2 tablespoons oil and lemon juice in bowl; season dressing with salt and pepper.
3. Place halibut on rimmed baking sheet; sprinkle with salt and pepper. Drizzle with some of dressing.
4. Bake until just opaque in center, about 12 minutes.
5. Meanwhile, cook pasta in large saucepan of boiling salted water until tender but still firm to bite; drain.
6. Add 2 tablespoons oil and garlic to same saucepan; sauté over medium heat 1 minute.
7. Add drained pasta, spinach, and tomatoes; stir to coat.
8. Season with salt and pepper. Remove from heat.
9. Cover; let stand 1 minute (spinach will wilt).
10. Divide pasta between 2 plates.
11. Top with halibut and remaining dressing.